Inosine (often referred to simply as the letter I) is a nucleoside that is very similar to adenosine; the only difference between the two is that in inosine, an -OH group replaces the -NH2 group in adenosine. It is not a normal nucleoside component of RNA, but it can show up in tRNA anticodons.

It must be noted that the wobble rules differ between organisms. For instance, in E. Coli, inosine in anticodons can pair with the bases adenine, cytosine, or uracil. This is not the case in the yeast Saccharomyces cerevisiae, where the wobble is more restricted. In this species of yeast, inosine can only be paired with uracil and cytosine.

See also: wobble, translation
Source: Essential Genetics: A Genomics Perspective by Daniel L. Hartl and Elizabeth W. Jones. is also interesting, for the curious reader.