Thinking of buying some cordless tools? First investigate the batteries. Why? Because replacement batteries often cost more than the bare tool! A Nicad battery may 'die' within 2 years but the tool has many more years of potential life. So, you will be faced with a $30+ cost to get a new battery to use a tool now worth $10 !
There is a solution : buy just one voltage of one popular brand and stick with only that brand/voltage/battery type. For example the Black&Decker 18 volt Firestorm cordless tools.There is a wide variety of common & not so common tools like impact drivers, hedge trimmers,sanders, yard blowers and the common drills in the Firestorm range. If you buy these as a set of 4 or 5 tools and then add specialty tools you'll probably accumulate 2 or 3 chargers and more importantly 6 or more batteries.Be careful to select the voltage based on the most demanding tool use you have.Some of the more unusual tools only come in one voltage. Be careful to choose the same voltage and the same physical size of battery. For example Firestorm come in 12v and 18v but are not interchangeable.Hitachi & Makita have two diffrent 9.6v batteries that aren't interchangeable.Yes, it gets tricky so be sure to physicaly see what you are buying - pictures on the web aren't enough.With some obscure brands (like Drill Master) from Harbor Freight you can interchange different voltage batteries on the same tool! Generaly batteries are not interchangeable between brands or even between voltages of the same brand. And of course the chargers are specific to brand+voltage too! There may be a choice or regular NiCad batteries or higher performance versions( both Ridgid and B&D have these). Whatever you get be sure to carefully rotate the batteries in use. Don't let some of them just sit in a semi discharged state. It is better to use them in sequence and recharge them immediately they are drained. Also ,don't leave them plugged in to the charger longer than necessary to fully charge them. Often a few hours or less is all they need.More charging time will slowly destroy them.
If you have a favorite cordless tool and it needs a replacement battery, you can rebuild them yourself.There are kits available & instructions on the internet& EBay. Alternatively get a generic replacement battery. For example the Makita 'stick' batteries (the 9000)  are about half price if you buy a generic.
Before you buy any new cordless tool, check their prices for replacement batteries .Also check that they are readily available. Some of the minor brands (like Hitachi for example) don't sell enough volume for your local big-box store to carry replacements.Some replacements are more than the bare tool is worth!