The Android Market's default search works well if you know exactly what you're looking for, but the keyword and other data in the store isn't particularly easy to search through. Appgravity is a search engine for the Android Market which offers more options for sorting and searching.
You can search Appgravity by a keyword, a publisher, or directly for the app you're looking for. Appgravity pulls its data directly from the Android Market, so you'll be sent to the official store if you find something you want to download. The added sort options and keyword searching means you'll get more results than the Android Market typically offers, so if you're hunting for an app but you're not sure of a title, you might get better results from Appgravity.