Measuring just 21x12cm, the Abus Granit Futura Mini is small enough to fit in the pocket of your jeans and strong enough to beat most thieves. It's light, too, and under 700g, and not too pricey either.
The entry price point for proper U-locks seems to get lower every year, and this unit from Master Lock at £30 (less online) is a great piece of entry level security, resisting attack with the tenacity of steel costing twice as much.
The Street Fortum is well constructed from hardened steel, with a shackle that goes right through the locking bar helping to anchor it in place. It's finished in orange plastic and frame-friendly rubber, and you get four keys and a fairly bog-standard frame mount that does the job without any fuss.
At just over £40 (less online) you'd expect mid-range performance from a U-lock but the Brute proved itself to be a tough cookie and it's a great lock to sling in your bag for the daily commute. You'll need a bag, as there's no bracket supplied, although one is available.
The Kryptonite New York 3000 isn't top of the range but is plenty tough enough to repel most criminals and is a good choice for everyday use where bike security is a necessity.
The New York features a 16mm hardened steel shackle and a disc-style cylinder with three keys supplied. The shackle locks at both ends into the base, and the lock mounts onto your bike with a metal sleeve on the shackle which slots into a resin bracket. The lock stayed put well over rough roads, and the LED key is a nice, if increasingly common, touch.
I want to buy a new lock and I don't know which one to buy. I have seen people with some sort of chain around there waists and that looks kinda cool plus I don't have to mount it on my bike. Are they crap?