AdSense remains the leader in on-line ads. However, there are many alternatives, targeted at different markets. Some have lower payout thresholds for example, and are better suited to smaller sites.
Buyer beware, not all alternatives come with stellar reputations. User experiences can be hit or miss. Read the fine print as some agreements have significant penalty clauses. Others may be licensed in offshore jurisdictions that are enforceable against you, but not the reverse if something goes wrong.