The Main Players are:

Alexa has a nice charting system. You can see your Reach, Rank and Pageviews charts and filter using date ranges and compare to other domains as well.
QuantCast gives you Traffic overview, US Demographics, Brand & Site Affinities, Similar Audience, Top Subdomains and traffic charts.

Here are some of the best tools to check your PageRank and more:

TagURLs is a web analytics tool combining the top 500,000 U.S. websites with their topic tags, using data from del.icio.us and QuantCast. Use it to discover the most trafficked sites for any topic of interest, such as “videos” or “education”, or by url, such seifi.org.

SEO Keys is a nice tool to search your domain for sites that link to you and their respective PageRank

Compete shows you a snapshop of your domain with a People Count chart and Top Keywords.

UrlTrends gives a trend report for your domain, that includes PageRank and Alex Rank, Incoming Google Links, Incoming Yahoo Links, Incoming MSN Links, Incoming Alexa Links and more.

Submit Express gives a nice table of your keywords, urls and a good analysis of your meta tags.

SEOmoz displays its own Page Strength and shows you details on how it calculates that score.

URL Investigator shows basic information such as Google Pagerank, Alexa Traffic Rank, WhoIs and Link Check data.

popuri is a simple Ajaxy tool that gives you the Google, Alexa and Quantcast Ranks, Google BackLinks, Yahoo BackLinks, Live Search BackLinks, Technorati Links, del.icio.us Bookmarks, Bloglines subscribers and more.

Statsaholic shows charts from Alexa and Quantcast for up to 3 domains.

WebConfs is another powerful and tool with lots of SEO checking utilities, such as Keyword Density Checker, Keyword Suggestions and more.

Bust A Name lets you mash up a series of words and shows you a set of available domains. In case you don’t know how to use web forms, Ryan has made a video nice little video demo for you.

Domain Scrub looks for typos and misspelled domain names. Most of the time these domains are baits for bad typers. Smart companies buy out all the typos in their name before anyone else does.

Domain hacks lets you find a domain that combines domain labels, especially the top-level domain (TLD), to spell out the full “name” or title of the domain, making a kind of pun, for example del.icio.us

MakeWords.com has advanced query features that allow you to refine your domain query for starting with, ending with, max letters, language rules random names and more.

Just Dropped is a domain search of recently dropped domain names, good for recycling/salvaging old domain names.

Psychic Whois, Instant Domain Search and PC Names. don’t suggest anything, but do give you instant availability for domain names as you type.

Domaint Tools offers a great suggestion tool for finding the right domain name, and also has a keyword based search tool with lots of options to narrow down your results.

squURL has a nice search interface and shows some simple suggestions as well.

DomainIt has a domain suggestion service to give you a large list of suggestions.

DomainsBot gives you an inline ajax auto complete feature for searching domain names.

Nameboy is another tool for using keywords to find domains

Yahoo domain search provides similar options in your search results.

Uwhois is a nice tool to search all Generic Top Level Domains and most Country Code TLDs.

Bemmu.com lists expired domains with indications of Wikipedia, part of speech, delete days, Google trends and more.

3LA lists Three Letter expired domain names, dyyo lists Four letter expired domain names, and 5 Letter lists 5 letter domain names.

Split It! visually shows you how your domain name string will be split. Use this to avoid getting an accidentally funny domain name.

Is your creative side running dry? Try these tools for some magic!

To come up with new words try Dislexicon

Word Mixer Enter 5 words and get 2-3 syllable results.

Word Finer for crossword pattern matching.

RhymeZone gives you rhymes and synonyms of any word.

Dot-o-mator is a tool for mixing letters and to make up your own words and make up a nifty Web 2.0 domain name.

WordConstructor is great for generating a random name, brand, or domain name.

A word of caution

Don’t get your idea stollen! Don’t search for your domain name until you are ready to register it. More companies are now selling their domain search records to people in trying to make money in lucrative ways. Read this article on domain tasting to find out more.

Do you know of any other unique domain tools?