Want UITableView to "snap to cell"

You can use the UIScrollViewDelegate methods on UITableView to do this:

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
    // if decelerating, let scrollViewDidEndDecelerating: handle it
    if (decelerate == NO) {
        [self centerTable];
    }
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    [self centerTable];
}

- (void)centerTable {
    NSIndexPath *pathForCenterCell = [self.tableView indexPathForRowAtPoint:CGPointMake(CGRectGetMidX(self.tableView.bounds), CGRectGetMidY(self.tableView.bounds))];

    [self.tableView scrollToRowAtIndexPath:pathForCenterCell atScrollPosition:UITableViewScrollPositionMiddle animated:YES];
}

There is a UIScrollView delegate method especially for this!

Edit: if you just want the code, look at the answers below which build off this.

The table view (which is a scroll view) will call - (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset when the user stops scrolling. You can set manipulate the targetContentOffset to ensure it ends up where you want, and it will decelerate ending at that position (just like a paging UIScrollView).

For example, if your cells were all 100 points high, you could do:

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset {
    targetContentOffset->y = 100 * (int)targetContentOffset->y/100;
}

Of course, you can also inspect the targetContentOffset passed in to see where it was going to land, and then find the cell that is in and alter it appropriately.

Building from what @jszumski posted, if you want the snap to occur mid drag, use this code:

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
    [self centerTable];
}

- (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView {
    [self centerTable];
}

- (void)centerTable {
    NSIndexPath *pathForCenterCell = [self.tableView indexPathForRowAtPoint:CGPointMake(CGRectGetMidX(self.tableView.bounds), CGRectGetMidY(self.tableView.bounds))];

    [self.tableView scrollToRowAtIndexPath:pathForCenterCell atScrollPosition:UITableViewScrollPositionMiddle animated:YES];
}

Extending @jesse-rusak's answer above, this is the code you would need to add to your UITableViewController subclass if you have cells with variable heights. This will avoid the double-scroll issue in the accepted answer.

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset {
    NSIndexPath *pathForTargetTopCell = [self.tableView indexPathForRowAtPoint:CGPointMake(CGRectGetMidX(self.tableView.bounds), targetContentOffset->y)];
    targetContentOffset->y = [self.tableView rectForRowAtIndexPath:pathForTargetTopCell].origin.y;
}