You can power most AC-powered devices directly from DC solar panel and avoid power losses in DC to AC inverter.

Apart from the solar panel, the rest of parts is very cheap. You probably already have them in your drawer.

It will work for most SMPSes (switching-mode power supplies), including:

laptop power supply units
desktop computer (ATX) power supply units
mobile phone chargers
single-board computer's adapters (like Raspberry Pi)
wireless routers's adapters
LED lamps

It will NOT work for:

transformer-powered devices
devices with AC motors, for example household fans, kitchen appliances

Step 1: Parts & Tools Needed

Parts:

solar panel(s) with enough voltage and power. For example, I have 2 panels, each with 50V output, connected in series. It gives approximately 100V which is enough for PSUs (power supply units) rated "input 100-240V". Some PSUs work with much lower voltage, even 40V. Read the section "Connecting the load" for more details.
electrical outlet (socket)
2 insulated wires (different colors recommended)
optional but highly recommended: 2 connectors matching your solar panel connectors, or crocodile clips
optional but highly recommended: case. I used a lunch box.
optional for low-current solar panels, otherwise required: fuse holder and fuse
optional: electrolytic capacitor(s). It needs to have voltage rating higher than peak voltage of your solar panels, and capacitance between 100 and 1000µF.

Read on for more explanation and parts recommendations.

Tools:

You can build basic version using merely screwdriver and wire cutters. Other tools you may find useful:

multimeter
soldering iron
files or drill
permanent marker

Step 2: Build the Basic Circuit

Strip insulation from both sides of both wires

On one end of wire, solder (or attach in other way, depending on connector type) the connector matching your solar panel's connector, or a crocodile clip. Repeat for the second wire.

If you don't have connectors or crocodile clips, just insert the wire with stripped insulation into solar panel's connector. I do not recommend this but it should work.

Connect the other ends of wires to the electrical outlet.

Connect the solar panel(s).

Measure the voltage in the outlet to make sure it works.

Do not plug in any device (except voltmeter) until you read "Connecting the load" section!

Step 3: Optional: Add Capacitor(s)

Capacitor isn't necessary, but it increases chance that a highly-demanding load will work, by increasing possible peak current.

Warning: Never measure solar panel's short circuit current when capacitor is connected! It may damage your amp meter.

Capacitor's voltage rating must be higher than peak voltage of your solar system. For example, if you have 2 solar panels with peak voltage (= open circuit voltage) of 63V, connected in series, the peak voltage of your system is 126V. Use 150V or higher capacitor.

Capacitor's capacitance should be between 100 and 1000µF. Too small capacitance and you won't have any benefit from it. Too large capacitance will make the circuit more dangerous because it will store more energy, so, for example, when you accidentally short the circuit, it will cause more sparks (don't do this).

You can connect several capacitors parallel or in series to "emulate" single capacitor. See this Wikipedia article for more details.

I got mine from an old ATX PSU. These are two 330µF electrolytic capacitors rated at 200V, connected in parallel for an "emulated" capacitance of 660µF.

How to connect the capacitor?

Important: disconnect the solar panel(s) before modifying the circuit!

Connect the capacitor(s) parallel to the solar panel and power outlet, observing polarity. Use a voltmeter if unsure! Wrong polarity may cause capacitor's explosion!

Step 4: Optional: Add Fuse

I highly recommend to insert a fuse in the circuit if your solar panel's short circuit current output, usually labeled Isc is higher than 2A. It will reduce chance of frying up circuits in case of accidents.

For example my panel has 1.42A which is fairly low, so the solar panel will limit current itself when shorted, that's why I haven't added a fuse.

Important: disconnect the solar panel(s) and discharge (short circuit) the capacitor before modifying the circuit!

To add the fuse, cut one wire between solar panel and power outlet, before the capacitor (if present), and solder the fuse holder there, then insert the fuse.

The fuse's rating should correspond to maximum power you want to draw from solar panel:

I = P / V

I - fuse's ampere rating

P - maximum power you want to draw, in watts

V - system voltage in volts

Note that it doesn't make sense to insert fuse with higher amp rating than short circuit current of your solar panel(s), because it will never blow.

Step 5: Optional: Case

Important: disconnect the solar panel(s) and discharge (short circuit) the capacitor before touching the circuit!

Make holes for electrical outlet.

Put the circuit inside.

Assemble the power outlet.

Close the case.

If you want, you can put polarity symbols "+" and "-" on the outlet's frame, using permanent marker or stickers.

Step 6: Connecting the Load

Now it's time to connect some load to your solar power outlet!

It can be any device with SMPS (switching-mode power supply), also called impulse power supply. They are designed for AC, but they'll work with DC. How? AC-powered SMPS is essentially bridge rectifier followed by DC-powered SMPS. When powered from DC, the current flows through rectifier's diodes in single direction and ends up on the same circuit as when connected to AC.

If your solar panel output voltage is below rated voltage of PSU (power supply unit), don't worry, try anyway. Many PSUs work with voltages below specification.

For example, you can try to connect device rated "Input 120-240V 50-60Hz AC" even if your panel(s) voltage is only 40V. Maybe it will work, maybe not.

On the other hand, never connect a device with maximum input voltage LOWER THAN solar panel voltage!

For example, don't connect 100-150V rated device if your solar panels system voltage is 200V.

If you're unsure whether your device has SMPS...

...it's best to experiment, but do it with caution:

Connect voltmeter to the solar panel(s)

Connect device in question to power outlet

If voltage falls down rapidly, disconnect the device as soon as possible. It means that it is either transformer-powered device (because transformers, unlike SMPSes, virtually short DC) or it draws too much power from your solar panel.

Step 7: Troubleshooting

If your PSU doesn't work at all, or starts and stops repeatedly:

try another PSU. Some work with lower voltages better than others
try lower power PSU. Maybe 2A phone charger is too much for the solar panel(s), but a 0.5A charger will work
if you want to power a laptop, reduce current drawn by closing all open applications.
insert the AC plug the other way. Some very cheap chargers use half-bridge rectifier, so they work with DC only when inserted one way.
ensure that solar panel is illuminated by direct sunlight. Diffuse light has much less power.
connect the PSU only first, and then powered device