Most of the possible options are listed above. Here's my take on each (caution, contains opinions)
As-is wireless - Fiddly to keep reliable over that distance without dedicated kit. Driving a tractor and trailer across the yard would probably cause the link to drop.
Wirelsss w. external antennae - Feasible, would require specific kit at each end and some configuration. A day's work plus the equipment costs, probably. Still some risk of interruption, but much less with careful siting.
Homeplug - Really, really simple if it works. No "sunk" costs, you can remove and re-site the kit easily.
CAT5 - risk of physical damage, not the cheapest solution, and some risk from different earth potentials between buildings, and lightning strike. Can be mitigated with surge protection, but this is becoming expensive. Definitely worth it if there is a need for Proper Bandwidth betwen the buildings, rather than enough to drive email and web browsing.
Fibre - the safest, highest-bandwidth option. You can buy a ready-made cable and a couple of converters, but again, it's not a particularly cheap option.
For that distance, I would try Homeplugs first. My own garage is connected to the house's main board with a longer cable than that. Works perfectly and has done for years. Buy a pair of the 200mbps ones from Argos and try it. (£50 for tp-link, £75 for Netgear)
If you rummage through some of my old posts you'll find numbers for distance and bandwidth acheived with 85Mbps plugs. More than good enough for general internet and email access, it's the ADSL that's the likely bottleneck.
edit - The reason I sugegsted Argos instead of somewhere cheaper was the low-hassle returns policy.
Edited by deleted (Mon 30-Jan-12 20:14:06)