I spend a good ten percent of every day watching monitoring tools. I have a handful of Dynamic Management View (DMV) queries and some stored procs that I like to run (I covered those in this previous post) but I also use three exceptionally helpful third-party tools. Two of them are free and one definitely is not. Still though I don't know what I'd do without any of them.
A happy SQL needs lots of disk space: tempDB, log files, databases, backups... there always seems to be -more- data growing its tentacles (never -less- oddly enough). As long as you're managing the storage allocation on the SQL boxes frankly you might as well be managing it across your entire shop, so you need a tool like Free Disk Management Software, FDMS. You need to be included in the Administrator group on each server; after that the setup allows you to select which disks you're actually interested in monitoring. Use the Options to set more appropriate bar-split color levels, and slow down the sampling to once every half hour. If you sort the display by free-space (low to high) then the problems obviously aggregate toward the top.
I've read several interesting posts on using the built-in Windows Performance Monitor to measure key SQL metrics, but to get serious you'll want to download this terrific free tool, Magnalis PerfPro, that wraps PerfMon in customizable MDI-mode windows with various display options. First though definitely perform an online search of "using PerfMon to monitor SQL" so you'll know which counters to add.
Finally, the semi-expensive monitoring tool that is worth every penny you spend: Redgate Monitor. This takes time to install and tweak properly, but with its excellent historical recordkeeping, variable instance-specified alerts, and flexibly filtered notification-clearing this tool will help you out with both your immediate concerns as well as your deep research.