You can play slide on just about any guitar, but some instruments are definitely more suitable for the purpose.
For acoustic slide playing, the instrument of choice is the resonator guitar (see more about these on my "Resonators" page). These instruments, commonly known by the trade name of Dobro (First patented by R. Dopyera in 1929), where originally manufactured during the 1920's by two companies, Dobro and National. They are distinctive for their metallic, haunting, tone. This unique sound is the result of having a metal cone (sometimes more than one) inside the body of the guitar which was originally designed to increase the volume of the guitar. This is usually concealed by a round metallic plate on the front of the guitar. Dobro and National guitars are produced both with timber and metal bodies. The wooden-bodied models generally have a warmer tone. These guitars, both vintage and new models, are expensive to buy. However a number of companies in Asia have been producing affordable copies for a number of years, of varying quality. Some of these companies include: Regal (originally produced in Chicago but now manufactured in Korea), Epiphone, Samick, Sakura and several other brands. These Asian copies are sometimes very hard to fault, although many suffer from having poor tonal characteristics due to their use of unseasoned timbers. Many of these guitars will improve tonally with age though.
Non-resonator guitars can make excellent slide instruments. Generally speaking, acoustic guitars with smaller bodies, as opposed to 'Jumbo' and 'Super-Jumbo' sized folk guitars have a tone more suited to slide playing. Also, as mentioned above, the age of the instrument's timber is an important factor in it's tonal quality. It is always important to remember that an instruments suitability for slide playing is completely unrelated to it's price! Hound Dog Taylor became a phenomenal slide player playing on guitars that probably cost him less than twenty dollars. Some very cheap guitars, both acoustic and electric, are excellent for the purpose. Another important fact is that a guitar used primarily for playing slide should have an action (The distance between the strings and the fingerboard) higher than what would be regarded as ideal for normal playing.
Another type of guitar popular with slide players is the "Weissenborn" (See picture below, right). These instruments were originally designed and manufactured by Hermann Weissenborn in Los Angeles in the 1920s and 1930s. Weissenborn have been popularised by players such as Ben Harper,
The best way to chose the guitar that is right for you to play slide on is to ask yourself "Who are my favourite slide players, whose sound do I like the best and what guitar are they doing it on?"
Opinions differ as to which is the best type of slide. Some players prefer glass slides, claiming they give a smoother, more mellow tone. Other players say that brass slides have more sustain and give a better "tinny" treble sound. As with selecting your guitar, selecting your slide is a matter of personal preference. Slide can be played using a Knife, a broken bottle neck, a beer can (full ones give better sustain !), a cigarette lighter, in fact just about any solid object weighing less than your average car could be used as a slide. However, a tube that fits on the finger you wish to use for slide-playing, that fits well without being too tight, is probably the best choice. Slides can be worn on any finger, it's really a matter of personal choice.