I think we'd all love to have a tool that made it easy to track those &^%$#$ MS licenses, but given the number of licensing scenarios, I think a roll-your-own database or spreadsheet is likely to remain a staple of license management for quite a while yet.
The way I look at it, there are two goals of "License Management," and you need to keep them both in mind:
Proof of ownership - if you were ever audited, do you have enough licenses and can you prove it?
Use of the software - for MS software, this is mostly a matter of managing keys and installation media
The thing is, you can use two very different systems for meeting those two goals and I think you can simplify things quite a bit by having some appropriate licensing strategies in place, such as:
You mention OEM licenses... We use OEM licenses on our PCs and we avoid license management issues by simply never upgrading the OS on a PC. We always buy a license with a new PC and that license stays with that PC until we dispose of it. Proof of ownership is built-in: stickers on the box
For servers and CALs, we always buy licenses and the available tools for managing those licenses are enough for us (but we're pretty small, ~20 servers, ~150 users).
For Office, we simplified things by not buying SA and upgrading as slowly as possible. We jumped from 2003 to 2010, and when we decided to upgrade to Office 2010, we knew we'd have to buy as many licenses as we were going to install it on. Proof of ownership is pretty simple (we have records of what we've bought) and we know how many are installed (which we've used various tools over the years to check). This is all done w/ License media, so key management is pretty simple.