Particularly for weddings with a lot of folks coming in from out of town, including a map along in your invitation can help keep your guests from getting lost. While a map printed on a little card is nice and all, it's standard and can get forgotten or misplaced. Map envelope inserts, however, are awesome, and ensure that your wedding guests will make it to your big day.
Please note: I'm not planning on getting married any time soon, but I'm thrilled to be able to share this idea/technique with folks who are!
If you have any questions, need any advice/assistance, or would rather someone whip these puppies up for you feel free to comment, or shoot me a note here or on Twitter.