I've had good luck finding overlays on 1) smaller sites that are trying to build a presence (Bodog, PokerChamps, Imperial, etc.) and 2) holidays.
There's a pretty good writeup on overlays on this site here...
http://www.tworags.com/?ACTION=overlays
My personal favorites are...
1) Bodog's Sunday $100K and $25K held at various times...both have fat overlays;
2) PokerChamps $15K held on Sunday...this tournament draws a small field of 75...the buy-in is $110 total and there's usually another $70 per entrant of guaranteed money in the prize pool. The site's actually pretty good; it surprises me that more people don't play the event; and
3) Absolute Poker's Saturday $100K...$162 total buy-in...draws about 550 entrants...there's a couple of bucks of overlay per person. This tournament used to be held weekly and had a nice overlay. They recently made it a bi-weekly event and the field got bigger, but there's still some juice here. Keep an eye on it; if it draw fewer than 617 entrants, there's extra dough.
Other smaller sites that have MTTs with overlays...Bugsy's, Titan, Imperial, etc. You can get a sense of which ones are good by looking at the guaranteed money and dividing it by the total buy-in (buy-in and fee). If the number you get is higher than the number of entrants, there's an overlay.
You should also try to play any of the major sites guaranteed MTTs on significant holidays. There are typically fewer entrants on these days resulting in modest overlays. For example, last Sunday (Easter), the MTTs at Bodog, Bugsys, Full Tilt, PokerChamps, PokerRoom, PokerStars, Pacific, Paradise, Ultimate Bet and Titan all had overlays. I noticed the same thing on Christmas Day '05 and New Years Day '06. Both fell on a Sunday and the big Sunday MTTs all had smaller fields than usual...and in many cases, a nice overlay.
Note that all of the above tournaments are "freeze-outs." There are a ton of guaranteed prize money rebuy tournaments, but I rarely see an overlay in any of them.
Hope this helps.
Ed