I use StashMyComics.com. Great service that is 100% free, no catches. Last time I used the comicpriceguide one, there was a 100-issue limit, and then you had to pay a fee if you wanted to add more.
Another one that is great, if you don't mind paying $30 a year, is ComicCollectorLive.com. I used it for years, until I found StashMyComics.com, because I like free better. And StashMyComics.com has a built in price guide, where the ComicCollectorLive.com one doesn't.