The following shenanigans not only demonstrate evidence of Longoria's awesomeness in the field, they're a good way to demonstrate that advanced fielding metrics don't bite.

I'm using both zone rating data from the Hardball Times and and ESPN, because their data come from different companies (BIS and STATS, respectively).  I'm going to compute runs saved compared to average for both data sets and then average them together.

A zone rating is simply the percentage of balls hit into a player's zone of responsibility that he turns into outs.  "Zone of responsibility" is pretty arbitrary, and is determined by combining all the little zone on the field that have their balls turned into outs at least 50% of the time (across all players).  BIS also reports out-of-zone (OOZ) plays made to the Hardball Times, which is great.  The best way to those is to count them the same as in-zone plays made and keep the denominator the same (balls hit into the zone).

An example of the BIS calculation (STATS is the same but without the extra OOZ step):

Evan Longoria has made 116 plays on balls in zone and he's had 160 balls hit into his zone.  His zone rating (or RZR as they call it) is just 116/160 = .712.  If you include Longo's 33 OOZ plays, his ratio is calculated as (116+33)/160 = .914.  This number could be above 1 (see Scott Rolen), but that doesn't matter.  Since the zones of responsibility are pretty arbitrary, the absolute rating has little meaning -- you can't compare a second baseman's zone rating to a shortstop's.  All we care about is Longo's performance number compared to the other third basemen.

The average third baseman has a ratio of .859 for the BIS data.  Longoria's obviously better than average.  To convert the difference into plays, calculate how many plays Longoria has made compared to how many plays the average third baseman would make in the same number of chances: 160 * (.914 - .859) = 8.96, or about 9 extra plays.  (That's on the same scale as John Dewan's +/- number, by the way.)

To convert to runs, we need to know how many runs each extra play is worth.  You can get really close just by comparing the linear weights value of an out and a single: .5 - -.3 = .8 runs (yes, that's half a run minus a negative three-tenths of a run). So Longoria's 9 extra plays are worth 9 * .8 = 7.2 runs.  That's about three-quarters of a win better than average.  Pretty good, although STATS has him just about league-average.

I ran the same calculations for each full-time third baseman, did it again with the STATS data, and then averaged the two.  (Since ESPN doesn't report ZR chances, I used the RZR chances to weight the STATS ZR.)  Here are the results:

NAME               STATS  BIS   AVG
Scott Rolen, Tor    10    21    16
Adrian Beltre, Sea   7    19    13
Chipper Jones, Atl   6    15    11
Jack Hannahan, Oak  -1    12     6
Blake DeWitt, LA     2    7      5
Joe Crede, CWS       3    5      4
Mike Lowell, Bos     6    2      4
Evan Longoria, TB    0    7      4
Troy Glaus, StL      2    4      3
Pedro Feliz, Phi     7    -4     2
David Wright, NYM   -3    6      2
Alex Rodriguez, NYY  2    1      1
Aramis Ramirez, ChC -4    4      0
Jose Castillo, SF   -1    -3    -2
Bill Hall, Mil      -1    -6    -4
Garrett Atkins, Col -1    -7    -4
Jose Bautista, Pit  -5    -3    -4
Melvin Mora, Bal     4    -15   -6
Kevin Kouzmanof, SD -4    -7    -6
Mark Reynolds, Ari  -5    -9    -7
Edwin Encarnac, Cin -5    -12   -9
Alex Gordon, KC     -8    -10   -9
Casey Blake, Cle    -4    -15   -9
Jorge Cantu, Fla    -7    -12   -10

If you're curious, the STATS runs and the BIS runs correlate at r=.65

Finally, as usual, you shouldn't use half a season's worth of stats to judge a player's true skill level.  In general, one year of fielding stats are as reliable as two year's worth of hitting stats.