United Airlines has several forms of electronic upgrades, but the most valuable (at least to those who travel internationally) are the Systemwide Upgrades (SWU). An SWU can be used to upgrade to the next class of service (economy to business, business to first) on any leg (regardless of length of number of flight segments) that United flies. My longest such upgrade was a part of my trip-from-hell where I used a single SWU to upgrade the LHR->ORD->HKG leg of 11,746 miles from coach to business.
However, there are restrictions (of course). In the old days, the SWU used to be able to upgrade any and all fares -- if you had a seat on the plan, you could upgrade it, provided an upgrade was available. Now, the upgrades, while still powerful, are limited to Y, B, E, M, U, H, Q, V, and W international economy fares (E-Upgrade Rules). This is still much better than the set of fares that you can use miles to upgrade: Y, B, M and H (United Upgrade Award Rules).
And, of course, it all depends upon availability. If the upgrade seat isn't available, you can't have it (but at least you can be wait listed to get the upgrade should it become available).
The next question one might ask is how does one come by these cool SWUs?
Unfortunately for most travelers, SWUs are only issued to Mileage Plus members who achieve 1K status (by earning 100,000 qualifying miles or flying 100 segments in a year). In January, each 1K member is given 6 SWU. In addition at each multiple of 50,000 miles above 100,000, the 1K member will be given 2 additional SWUs (so at 150,000 miles you get 2 SWUs and again at 200,000 miles).
There is one other time that you get SWUs -- when you cross one million lifetime flight miles, United gives you 3 SWUs.
There is a gray market for selling SWUs on the likes of eBay. Of course, they will all claim that they are not selling you the upgrade itself, but an envelope that contains the upgrade -- that way the seller hopes to stay within the rules United has against selling the upgrades. The price of the upgrade varies with the amount of time left on the upgrade (they only last 1 year) with an average of around $400.
I have never sold or bought an upgrade (really -- I'm not just saying that for United's eyes), so I can't tell you how good it works. I have given upgrades to my family members and my sister has shared some of hers with me, so I presume it would work fine -- I would be less than comfortable with providing a stranger with my itinerary details so that they could apply the upgrade.