There are a lot of places in the United States where weed is already, for all practical purposes, legalized unless you possess large quantities. In Portland and I believe Seattle if you are under a certain amount on your possession (no clue what that amount is) they just take it away from you. In other words, they use weight to differentiate between users and dealers and only really penalize dealers.
If we took the drug offenders out of jails and court systems, we could use those for actual crimes that involve victims. Imagine people who steal your crap getting locked up for a substantial amount of time - that would make me happy. Also we would save money on jails, maybe we could put that money into schools or something...
Supposing that pot was legal though:
-Sales would be more easily regulated. As it stands now, it is easier for kids to get pot than alcohol...so guess which one they get?
-DUI could still remain in effect. If you are driving erratically and they test positive for THC in your urine, then you would be considered to be driving under the influence, whether you smoked a joint 10 minutes ago or 10 days ago.
-The military would still set their own policies for it. And really, this applies to all businesses. Places of employment would STILL probably use urinalysis. It would wind up being like other things that are practically legal, but kept in check by the free market and the users need to be able to hold a job.
-The government could still tax marijuana sold in stores. Some people could/would grow it on their own, but the majority of people probably aren't that dedicated to their drugs, they would just buy it at the store.
The US Govt would save money on many different levels: War on Drugs, Jails, Courts, Tax Revenue generated, etc...
<- I'm not a pothead, I am just a libertarian. I am also in favor of allowing people to smoke cigarettes in public (the horrors) and I am also in favor of letting people have whatever guns they want to have.