ask.com and snap.com both offer preview images of some websites in their search results. Unlike SearchMash, these are pre-rendered thumbnails, so you can't click on them, they only have them for the most popular sites, and they may be out of date.
Google Preview is a free Firefox plugin that provides similar functionality to ask and snap's thumbnails, but pulling the images from thumbshots.com and alexa rather than a proprietary database.
Browster is an ad supported browser plugin, and the only other one that does give you a live preview of web pages. As a browser plugin, it does require an install, doesn't do any search term checking, and is only available for the PC. The upside is that as a plugin, the integration is tighter than SearchMash.
For more mashups, ProgrammableWeb has a great directory, and recently gave SearchMash four stars.