I know of Havelock and NXT, but what are some of the other popular stock platforms available.
XCP ,MSC , HZ and may be NEM are other options.
NEM's asset exchange isn't live yet and probably won't be until a few more weeks at least. And I wouldn't exactly call HZ a "popular" stock platform. It's a NXT clone and most NXT clones (e.g. FIMK, BURST) have asset exchanges which are hardly used. Qora has an asset exchange too although nobody uses it.
Also, BitShares has support for user-issued assets. Again, I don't think it's widely used. NXT is definitely the most popular one with XCP and MSC probably being the second and third most popular platforms, respectively.
For a more comprehensive analysis of decentralized cryptosecurity issuance platforms, check out Overstock's wiki:
http://o.info/index.php/How_to_issue_a_cryptosecurityHere is a list of guides for each platform in case you want to issue your own cryptosecurity:
http://o.info/index.php/Category:Cryptocurrency