I've taken several approaches at different times and for different styles of games.
1. I'm really rather keen these days on the 'Preparedness' skill of Ashen Stars and Maschine Zeit. Players suddenly decide they need a sonic screwdriver, a tennis ball or a pair of flip flops? Then they roll Preparedness and if they succeed they've got the equipment. You're not allowed to claim you've been carrying a grand piano around with you for the last 2 days, but you can (if appropriate) say that FedEx has just delivered one.
2. When running a Traveller style 'space traders' campaign using GURPS, we used what we referred to as 'the GURPS Argos catalogue'. For non-Brits, Argos is this: http://www.argos.co.uk/static/Home.htm
. Rule was, if it was in the Argos catalogue, just cross off the money and add it to your character sheet. This prevented lots of whining to the GM about equipment and long pointless conversations about what a far future tennis ball or pair of flip flops would cost. (Although the PCs spaceship did end up with rather more plastic lawn chairs and BBQ sets than might be expected in your average space traders game...
3. For military games, they get a standard list of kit that they are issued - everything from weapons to how many days worth of rations and toilet paper they are given for a 'standard' mission-as-game-session. Then the specialists get their special stuff - medical gear for the medic, etc. They also get a sheet with all the standard weapon stats listed on it, so if they drop their SA80 and grab their mate's sniper rifle or an enemy's AK-47 there is no faffing about with rulebooks.