There are many online tools that can be very useful for webmasters. I went through my bookmarks and am listing some that I use. Enjoy!

Spider Test Tools: Shows the page size, source code, meta tags, page title, textual copy of the page, keyword breakdown by density (including 2 and 3 word phrases), number of words, number of unique words, and outbound links on a page.
PC Names: Various tools here. Domain Check checks the availability of a domain as you type. Word Search searches through all available domain names that contain a word you enter. Domain Suggest gives you alternatives for the domain you want. There are other tools there as well. This is a great site for easily finding a domain name based that is available based on words you enter.
SEO Tools: 17 tools relating to SEO. This is a good one!
Htaccess Tools: Various tools for creating .htaccess files. Great tool.
HTML Basix: Htaccess and html code generators.
FavIcon Generator: Generates a favicon from a graphic file you provide.
Webmaster Tool Kit: Offers a ton of online tools for webmasters.
Pausing RSS Scroller: Have limited space on your website? Use this tool to get more out of less. Not easy to setup, but very useful.
Random Iframe Generator: Randomly rotate text, HTML, or banners.
Wepage Tools: Membership site that offers some sample free tools.

If you really want a killer list of online tools, then check out Smashing Magazine’s blog post of online tools. It just begs to be bookmarked.