I use SQL Monitor from RedGate as it comes as part of a package I have but it can be bought on it own. I also use Confio Ignite Free, because its free and I cant afford to buy any other products! I like to see them both highlight the same issues so I am sure I'm not chasing smoke.
RedGate are busy building the next version of SQL Monitor which is very different from the previous version. You can get a free trial of both from their site although SQL Response v2 is currently a Beta version.
Disclosure: I am a Friend of RedGate so am involved early on in their product development on some tools. I have been testing and trialling SQL Monitor v2 for several months and would have a bias towards using that.