GLBSE is a website where companies and individuals can issue and sell stocks, bonds, and options via Bitcoin. Anyone can create an account and issue any contracts he wants - a contract is just some text, signed by him (For example I created this bond as a proof of concept where I promised I will pay X amount of Bitcoin in the future for a loan of Y amount of Bitcoin today).

You can use GLBSE as an investor, transferring Bitcoins there and placing BUY and SELL orders on the various assets, or you can use it to sell parts of your company. It is a small scale version of a stock exchange (e.g. Nasdaq), with much lower fees and (almost?) no regulation. To create an IPO at an official stock exchange would probablly cost you at least a few thousand dollars, on GLBSE you can do it for half a bitcoin.

Although GLBSE has a command line client and an API, the easiest way to use it would be the javascript client. You can see a list of assets here, and follow this blog for posts about new assets. An updated list of transactions is available via Twitter, and you can find graphs of the assets prices here.