Discrimination comes in any variety of forms, given the diversity of today's workplace in age, gender, race, ethnicity, language, etc.
I think age discrimination is one of the more common forms of discrimination today, and one that is harder to enforce equity laws for. As the Baby Boom generation ages, two things happen: 1) they start to lose some skills that would make them more employable, and 2) having been around the workplace longer, with typically more education and experience, they are more expensive. For both of these reasons, there is a temptation among business owners and managers to replace older workers. While age might be their main motivation for this, they can find other reasons - downsizing, outsourcing, etc. - to lay off the person instead of fire them. This saves them more money and gets them around the discrimination laws.
Gender discrimination is still quite common, though less so than in decades past. Women still are, on average, paid less than men, and occupy less of the top business and management positions.
Discrimination by race, of course, is still commonplace, though it varies for different races. It has become more difficult, socially and legally, to discriminate against African-Americans and Asian-Americans in the workplace, while discrimination against Latinos and other recent immigrants is widespread.
Discrimination against the gay and lesbian population is still quite rampant, socially acceptable and legally difficult to prove.
There are more examples, of course, but I think this covers the bases pretty well.