You should always know where your site is ranking, or not ranking if you’re one of the unfortunate… Here are a few tools I recommend:

Searching the top 50 SERPS, this tool at SEO Bench will Check for rankings on Google, Yahoo! and MSN at once.
http://www.seobench.com/search-engine-rank-checker/

There are many many different Google datacenters, and each of these datacenters have the potential to respond with different results for the same search query. Most times, seeing results that vary from datacenter to data center, means that Google is in the middle of updating their search index. Check out the Multiple Datacenter Rank Check Tool here:
http://www.seologs.com/dc-rank-check.html.