There's currently no replacement for a traditional printed price guide, of which there are a number on the market. However, there are many resources online which will help you research your collection. Here are a few:
Tracks online sales of high-value records.
Articles, trivia and forums for collectors.
Clearinghouse with an absolutely huge number of record- and music-related links.
Another clearinghouse with links to a variety of
record collecting websites.
Records for sale from numerous vendors.
Still one of the best places to start when researching current market values. Check the Discussion Boards for tips on grading and evaluating your records.