I have used Restaurant.com many times. Search by zip code and narrow it down by how many miles from that zip code you are willing to travel. As far as "Disney" restuarants, are you looking at Florida or California? I purchased a GC for The House of Blues which is in Downtown Disney in California.
All you have to do is present the printed Gift Certificate when ordering.
Read the fine print before you purchase! Some require you to spend a certain amount, usually $35.00, then minus the $25.00 gift cert. (not including alcoholic beverages). Others are limited to certain days of the week (example: Mon - Fri only or not valid on Sundays, etc.)
Give it a try. Sign up for the e-mails because you can get some really great deals. I have gotten a $25.00 certificate for $2.00!
Yes, they do work great, as the PP said - read the restrictions, each restaurant has different ones. I used 2 last week for 2 places while on vacation, easy as pie I have already bought more for another trip.