ping -a w.x.y.z

Should resolve the name from the IP address if the reverse lookup zone has been set up properly. If the reverse lookup zone does not have an entry for the record, the -a will just ping without a name.

You can use dig +noall +answer -x <IP> to look up an IP address.

To simply loop over a file that contains a list of IP addresses:

while read ip; do dig +noall +answer -x $ip; done < ips.txt

Or, pipe the output of your counting command. This time we get the count and the IP addresses separately and then print them on one line:

cat access.log | awk '{print $1}' | sort | 
uniq -c | sort -n | tail -n10 |
while read count ip; do printf "%d " $count; printf "%s " $ip; dig +noall +answer -x $ip; done

Example (sorry for the UUOC):

cat test | while read count ip; do printf "%d " $count; printf "%s " $ip; dig +noall +answer -x $ip; done
20 8.8.8.8 8.8.8.8.in-addr.arpa.    52767   IN  PTR google-public-dns-a.google.com.
22 8.8.4.4 4.4.8.8.in-addr.arpa.    61369   IN  PTR google-public-dns-b.google.com.

You can further pipe dig's output into awk to just get the host name:

cat test | while read count ip; do printf "%d " $count; printf "%s " $ip; echo $(dig +noall +answer -x $ip | awk '{ print $(NF) }'); done
20 8.8.8.8 google-public-dns-a.google.com.
22 8.8.4.4 google-public-dns-b.google.com.

The commands dig and host should be what you're looking for ;)

On *nix systems, you can use this command:

dig -x [address]

Alternatively, you can add +short at the end of the dig command to output only the DNS result.

There's also nslookup on both *nix and Windows systems for reverse DNS requests.