Explorer

A good all-rounder product.

Free incoming calls in over 40 countries.

Low outgoing rates.

Very few!

Choose this or the Passport depending on the countries you're visiting.  There are very few differences between the two SIMs.

Passport

recommended for travelers planning to use their phone a moderate amount

Free incoming calls in 50 countries.
Low outgoing rates.
Low renewal cost.
The SIM's phone number is conveniently based in Britain making it easy and inexpensive for people to call to you.

Very few!

Sometimes this will be better than the Explorer in terms of calling rates, sometimes not, it really depends on the countries you expect you'll most commonly be using your phone in.

Passport Plus

Free incoming calls in one country (British Virgin Islands).
Lowest outgoing rates of the three Passport SIMs, but you pay for incoming calls too.
Low renewal cost.
The SIM's phone number is conveniently based in Britain making it easy and inexpensive for people to call to you.

Paying for incoming calls just about everywhere

Lowest outgoing costs, but you pay for incoming calls too.

Passport Light

good for the very low volume user as an alternate to Mobal

Free incoming calls in 4 countries (Britain France Germany Italy).
Highest outgoing rates of the three Passport SIMs.
Low purchase cost - $19, including $20 worth of call credit
Low renewal cost.
The SIM's phone number is conveniently based in Britain making it easy and inexpensive for people to call to you.

Very few!

People who travel maybe once or twice a year and want an emergency number that they won't use much.

Mobal

recommended for infrequent travelers who don't make lots of calls

A lifetime phone number with no ongoing fees or renewal costs or hassle at all.
No prepayments or other fees, easy billing direct to your credit card - you only pay after you've actually used the SIM.
The SIM's phone number is conveniently based in Britain making it easy and inexpensive for people to call to you.

Expensive calling costs
Pay for all incoming calls except in Britain.

Occasional travelers primarily wanting a number for 'emergency' use.

Riiing

Now in two different versions - confusingly, both of which seem almost exactly the same as each other

Free incoming calls in 80 countries.

Outgoing calls are expensive and have a setup fee as well.
SIM expires if not renewed every 9 months.
Costly minimum renewal amount.

People planning to use their phone primarily to receive incoming calls and who will be traveling to countries where Explorer and Passport service don't also offer free incoming calls.

We have no opinion which of these two products is better than the other - the difference is very small.

Hop

Very simple billing, you always know how much calls cost you no matter where you are.

Calls charged in one second increments.

No voicemail.
No free incoming calls anywhere.

This product has withstood the test of time for a long time.  It doesn't work well with all phones, though, and the newer products (Explorer and Passport in particular) may be better value, too.

Local SIM

recommended for business travelers and people traveling to one country for an extended period

Usually free incoming calls in the country the SIM is registered to.  Easy for locals to call you.
Usually cheap local calling rates.

A different number for every country makes it harder for people to conveniently know how to contact you.
Will expire if not regularly used.

People staying in one country for an extended time, people planning to use their phone a lot, people wishing to be easily contacted by other people in the country they're visiting.

International roaming with your current (eg US) cell phone

Nothing extra to do or change, keep your same number as at home.

Will only work if you have an international compatible GSM phone (and, in the US, service with either Cingular/AT&T or T-mobile). Pay for incoming and outgoing calls at rates that can be a lot higher than for other options, pay for voicemail, may get expensive unwanted calls from people calling your regular number expecting you to be at home.  Requires locals in the country you're visiting to make an international call to reach you.

Short infrequent journeys with very little planned phone usage.  People needing data service.