Save money and keep your bike running better by doing your own maintenance; here's what you need.
You don't need a wide array of specialist tools to work on your bike. Most essential jobs can be done with a few good quality standard tools and a handful of bike-specific ones.
Ball-end Allen keys. Don't skimp on these; you'll be using them a lot. Ball-end keys allow you to turn a bolt from an angle, which speeds up many jobs. As well as being harder and more accurately made, and therefore less likely to mash the bolts you tighten with them, high-quality keys have a narrower neck for the ball, and therefore work at steeper angles, making them more versatile.
Screwdrivers. You want a couple of flat-blade screwdrivers and Phillips (cross-head) No 1 and 2, and possibly a size 0 too. A more extensive set will include sizes that are useful round the house too.
Combination spanners. I almost hesitate to include these because bolts with spanner flats are now rare on good quality bikes. You will almost certainly never need more than 8, 9 and 10mm, plus a 13mm if you have bolt-up hubs. If you need spanners for other jobs, then the sets we've suggested have everything you need for the bike too, but if bike fettling is your only need, then it'll be cheaper to buy individual spanners.
Pliers. A set of combination pliers has lots of uses, from generally holding and pulling parts to crimping cable ends. You'll also find lots of uses for long-nose pliers, so a set of three with side cutters is good value.
Tyre levers. You need a couple of sets, one for your home toolbox and one for your on-bike tool bag.
Floor pump. It's much easier to keep your tyre pressures up to snuff with a floor pump (aka a track pump) than any portable pump.
Pedal spanner. If your pedals have 15mm flats, then you'll need a 15mm spanner to take them on and off. A standard 15mm spanner will fit some pedals, but others need the thinner jaws of a specific pedal spanner.
Cable puller. Owners of hydraulic-braked bikes with electronic shifting can ignore this. The rest of us will find fitting and adjusting brake and gear cables a lot easier with a tool that pulls the cable snug and holds it in place while you tighten the clamp bolt.
Cable cutter. Do not try and cut cables with pliers, sidecutters, tin snips or any other vaguely sharp tool you have kicking around; you'll just make a mess of them. Get yourself a proper set of cable cutters with blades shaped to keep the cable strands together.
Chain wear gauge. You can keep an eye on the wear of your chain by measuring its length over 12 full links with a good quality ruler. If it's 12 1/16in long, then it's time to replace it and if it's reached 12 1/8in you will probably have to replace the sprockets too. A wear gauge makes this easier by telling you when your chain needs ditching.
Workstand. On the one hand, this is a bit of a luxury; on the other being able to hold your bike steady and well clear of the floor makes any job easier. Your back will thank you for not leaning over a bike for hours on end too.