I've used several products over the years so here are a few of the more popular examples for returning your lead miniatures to bare metal ready for repainting. Some of the products listed may not be available in every country so I have tried to give generic names or alternatives where possible.
Pinsol / Pine oil- Good on Acrylics and Enamels. Soak miniature for at least 6 hours, longer if possible & remove with a soft brush. may require several applications. Bets not to use this on plastic models but can produce good results removing acrylics from metal after only 2-3 hrs.
Castrol Super Clean - A non smelly product that is also bio-degradable and is safe to use on plastic models as well. This product contains Sodium Hydroxide and can produce excellent results after only 6 hrs. The main drawback is that this product comes in a gallon can which is probably more than anyone needs for this type of job.
Simply Green - A 2-Butoxethanol based product that can be used on Plastic or metal models. Best used as a soak rather than sprayed or brushed on. The product is Non Toxic, Biodegradable and non corrosive. Gives good paint removal after only 6 hrs but for best results soak for 12 hours.
Oven Cleaner - This is another product that uses Sodium Hydroxide as its main active ingredient. There are lots of different products available but the one that get mentioned often in various miniatures/painting forums is Easy-Off Oven Cleaner. Soaking produces the best results after 6hrs.
Brake Fluid - Can be bought in relatively small quantities from any garage. Brake Fluid gives best results after 24hrs soaking so this isn't a fast option. Safe to use on plastics but needs to be disposed of responsibly as it is harmful to the environment.
Dawn Power Dissolver - Recommended by the guys at From the Warp this product can produce results in a little as 20 minutes. Because it is a foam it can be sprayed on to the model and left to do its stuff, but be careful not to spray it onto anything you don't want stripped!
Nail Polish Remover / Acetone - Easy to acquire but smelly as hell. Use in a well ventilated area and soak your model for at least 6 hrs. Do not use on plastic miniatures unless you want them to turn into slag.
There are several other products that I have read about, but the application and results are (allegedly) broadly the same. Some of these such as Linseed Oil and regular DIY store paint stripper are fairly logical options. Others like Lighter Fluid sound distinctly dangerous and then there are the weird and wacky options such as Coca-Cola and Distilled Water.
Aside from the paint removing medium you will need some or all of the following tools to clean your models. Most people won't have a problem with any of the products listed but some may have a chemical allergic reaction so its always a good precaution is to wear some rubber gloves. A ceramic or metal container to soak your miniatures in. An old toothbrush is also essential. Regardless of what method you use an good scrub with a toothbrush can help lift loose paint and get in those hard to reach spots. An absorbent cloth or paper towel to mop up spills... trust me this is essential. A small bit chamie leather to grip the model with while scrubbing. This is soft enough not to damage the casting and can be washed out between uses. The tool of last resort is the dental pick which lets you get in the deepest recesses and remove those stubborn bits of paint.