The Cubs had the second choice in the 2012 Rule 5 Draft, and chose Hector Rondon, a right-handed pitcher, off the Cleveland Indians' Triple-A roster. Some here had hoped the Cubs would take Josh Fields from the Red Sox, but the Astros took him with the first choice. The Cubs lost Starling Peralta to the Diamondbacks; that's the only loss from the Cubs organization at the time I made this post.
Here's what the Rule 5 Draft is and how it works (via Wikipedia, more at the link):
As in the amateur draft, the selection order of the teams is based on each team's win-loss record from the prior regular season, each round starting with the team with the worst record and proceeding in order to the team with the best record. Any player selected under Rule 5 is immediately added to his new team's 40-man roster; thus, teams who do not have an available roster spot may not participate in the Rule 5 draft. Players who are not currently on their team's 40-man roster are eligible to be selected in the Rule 5 draft, but only after a standard exemption period has elapsed. See Selection eligibility below. If chosen in the Rule 5 draft, a player must be kept on the selecting team's 25-man major league roster for the entire season after the draft—he may not be optioned or designated to the minors. The selecting team may, at any time, waive the Rule 5 draftee. If a Rule 5 draftee clears waivers by not signing with a new MLB team, he must be offered back to the original team, effectively canceling the Rule 5 draft choice. Once a Rule 5 draftee spends an entire season on his new team's 25-man roster, his status reverts to normal and he may be optioned or designated for assignment. To prevent the abuse of the Rule 5 draft, the rule also states that the draftee must be active for at least 90 days. This keeps teams from drafting players, then placing them on the disabled list for the majority of the season. For example, if a Rule 5 draftee was only active for 67 days in his first season with his new club, he must be active for an additional 23 days in his second season to satisfy the Rule 5 requirements. Any player chosen in the Rule 5 draft may be traded to any team while under the Rule 5 restrictions, but the restrictions transfer to the new team. If the new team does not want to keep the player on its 25-man roster for the season, he must be offered back to the team of which he was a member when chosen in the draft.
Use this post all day for Rule 5 discussion.