I recently switched from a Microsoft Action Pack subscription to a TechNet Plus subscription. The two subscriptions share the general purpose of allowing you to test and evaluate a wide variety of Microsoft products without paying full price (or anywhere close to it). Signing up for TechNet Plus was much simpler than the Action Pack.  It seems anyone with a Hotmail account (Windows Live ID) and a credit card can apply. As with the Action Pack, you get a ton of Microsoft software with no time limits. You get keys and activations with most just as you would purchasing the software retail (in most cases, you get a key that allows up to 10 activations, as with Vista or Office). In both cases, when your subscriptions expire, you're no longer legally able to continue using your software (although the activations will not deactivate, more of the honor system). Some differences are:

1) TechNet is for testing/evaluating only, where the Action Pack is supposedly okay to use internally in the business (depending on who you speak to, I guess).

2) TechNet offers more software and versions.  Example, in the Action Pack, you get Vista Business (10 licenses) and 1 copy of Vista Ultimate.  With TechNet Plus, it seems you get to download Vista Business, Enterprise and Ultimate, all having a separate key (I'm assuming 10 activations each).  However, it's been unclear in any documentation so far as to whether or not all 3 versions share the same 10 activations.

3) TechNet typically costs $199 for the Standard download-only option, but is $149 for yearly renewals. TechNet Professional is $349/yr with $249/yr renewals. I believe the Action Pack is $299 every year.  The TechNet Plus option to have discs mailed to you costs $599. As you can guess, I went w/ the download-only option (downloads are ISO files which you burn to discs afterward). While I'd much rather have nicely factory-made discs over blanks w/ Sharpie labeling, I can't justify the price difference.

Promotional Codes come and go for the TechNet Plus subscription.  I took the bait on a recent one mentioned at a MS event: 40% off, which dropped the $349 to close to $210. The code I used was:
TMSAML03 (sorry, expired 6/30/08).  There are others floating around for as well.  Be sure to try Googling some phrases like Technet Promotional Code and see what pulls up.

TechNet Plus Direct Full Online Subscription All Languages Regular: $349.00
Use promo code TNITE13 for 28% off. (This offer is for new orders only and valid only in the U.S. until March 31, 2010.)

Update:

TechNet subscriptions have changed to Standard and Professional.  The Professional is basically the same package as before under the TechNet Plus name.  The Standard excludes some of the more enterprise-geared software, but it only costs $199 for new subscribers and $149 to renew!

Try promo code TNFLA12 for 15% off a new Professional subscription (Expires 2/29/2012).

Standard vs Professional Comparison Chart

Full list of products included (Excel file direct from MS)