This free online tools can check backlinks for your website, the essential tool for every webmasters since backlinks is the ultimate key to SE traffic.

This service searches for duplicate content all over the net. Useful for checking sites then you want to do some link exchange or find out if someone has stolen your content.

This tool allows you to compare two URLs for duplicate content percentage - you can use it to determine the chance that google would penalize you for duplicate content posting.

This great tool can dig you website and check the density of keywords. As you may guess you can use it for SE optimization of you site ;)

Reliable tool for google PR checks - multiple servers

Simple and quick PR check tool - mostly I use this one.

This one is really great! I use it like several time during the day and in my opinion it is the best free keyword parser. Using this tool you can determine number of request for every keyword you want to use and choose the most appropriate and one. Use it together with Google Adwords

Another irreplaceable tool in the inventory of each SEO. Check the competition for keywords that found using Keyword parser and find out new ones

Free keyword suggestion tool with wide range than wordtracker, however I would recommend to compare results with Wordtracker. Ideal for researching terms with low searches per day, so I find it quite useful