Friends with benefits?

Solution 1:

The friends are:

• Roman - You can call him to get a taxi ride for free.
• Little Jacob - He will show up and sell you guns at a discount.
• Bruicie - You can call to get a helicopter to take you elsewhere in the city
• Dwayne - Will send you a couple of armed "homies"
• Patrick - Sends you a car bomb

There are also some girlfriend options with benefits, but only those that you meet via the internet. As far as I know, the "storyline" girlfriends have no benefits.

• Carmen - She will give you a health boost ("medical advice") if you call her
• Kiki - She can remove wanted levels (up to 3*)
• Alex - Call her to get a discount on store purchases (50%)

Once you know these friends, you'll have to build their relationship level. You accomplish this by calling them (or answering when they call) and doing something they're interested in. The GTA Wikia has pages for friends and girlfriends that list their preferred activities.

Once you have the benefit, the easiest way to keep it is to always accept their requests when they call. Refusing their requests will cause a relationship loss. However, (at least at the time I played it - this may have been patched) you can call them back and cancel immediately for no penalty.

Some of these friends can be killed as a result of your decisions in the game. I won't spoil it and tell you who, (the linked Wikia articles will, though!) but clearly if you value a benefit you will want to think carefully when you're given the choice.