Readability bookmarklet: Open any Indian news site like rediff or times of India or Economic Times. If you want to read any article, it is very distracting. One very good bookmarklet which can make your life very easy is 'Readabiliy'. You can customize how you want to see the page and it will be rendered like that. No ads no distracting flash. Try one and you will not look back.
bit.ly sidebar : You are reading a good article and you want to tweet it to your friends. You can press the retweet button on the site or go to twitter copy the url and send. But in the first case, there is no easy way to enter your comments and in the second case, the url is very big so you cannot write your comments. This bit.ly sidebar opens on the side after you click with the url shortened. You can enter your comments and tweet it.
Share on Tumblr: You are reading something interesting. You wanted to save few lines of it for yourself. You can do that with tumblr. You can have a tumblog and using this bookmarklet save text, pics and videos to tumblr directly. See my tumblog here. Visit it for amazing quotes.
Add Zemanta: This is a helper in your blogging. Suppose you are blogging and how good it would be if somebody directly help you link important text, related blogs, contextual images and good tags. That is Zemanta for you. If you are a blog, this will surely help you.
Delicious: The best bookmark manager of the web has a bookmarklet too. You can save your bookmarks by just clicking a button and after saving you will be redirected to to the original page. And whats more, you can even tweet directly from there.
Read Later: If you have opened too many windows for reading and suddenly you feel that you don't have sufficient time to read all of them now you can save them later for reading and read them later. There is another site or ReadItLater also which is also quite useful. But I like the Instapaper's presentation better.
Try them out and suggest any good bookmarklets that you use.