T-Mo UK--a useful option
 

I've had a T-Mobile UK SIM for a long time (along with others) since I travel often from the US to the UK. It's not a discount brand but it's become more affordable over the last year or so for two reasons:
1. The "Everyone" price plan, which prices all domestic UK calls at 12p (about $.24). This is pretty good in the market; some others price "same-network" calls cheaper but I call lots of different numbers over there so this appeals to me.

2. Cheap bundles of minutes for specific countries: £2 ($4) for 50 minutes to US or Canada, or £7.50 ($15) for 50 mins to several Asian countries. If you use the whole bundle back to the US, that's only $.08 a minute, which is hard to beat for a mainstream, no-ringback carrier. (I also have an Orange UK SIM, which is $.40/min to the US; without the bundle, the T-Mo rate to the US is a whopping $1.80/min!).

Not perfect for everyone, but it's what I need: reasonable domestic calls, cheap calls to US, free incoming voice and text, cheap texts outgoing; AND there's no monthly charge or connection fee, with your minutes non-expiring as long as you make a chargeable call or text once every 6 months (easy even if not in the UK since they have roaming everywhere though at a high price). Also good voicemail and customer service.

you might convert your Orange SIM to Call Abroad at 5p to USA

The T-mobile offer lasts up to 90 days per bundle, and says it is limited to 6 per person, but you could always get another SIM

Mobile World is also 5p to USA

O2's int'l bolt-on is cheap for Europe, and might be used to USA via calling card access in Ireland

It's a standard in Europe - nobody here considers it as any advantage :). Good deal is something more e.g. free voicemail or free incoming calls abroad...

I was comparing it to the global sims, as well as to the mainstream carriers who charge for incoming voice and text when roaming.

Global sims never charge for incoming texts (including the case SMSing doesn't work at all :-P). So do mainstream European carriers - charging for incoming SMSes abroad is very, very rare. Some of them don't even charge for incoming MMSes...

Mention these bundles on the TMO UK page
 

It would be a good idea to mention the bundles the first poster talks about on the T-Mobile UK page on this site, if only because TMO really doesn't seem to be a terribly good deal to most international users without them.