It's useful to use 'useful' instead of 'usefull'

plugins-scripts/check_ifoperstatus.pl

@@ -325,7 +325,7 @@ sub print_help() {
 	printf "                     (Implies the use of -I)\n";
 	printf "   -w (--warn =i|w|c) ignore|warn|crit if the interface is dormant (default critical)\n";
 	printf "   -D (--admin-down =i|w|c) same for administratively down interfaces (default warning)\n";
-	printf "   -M (--maxmsgsize) Max message size - usefull only for v1 or v2c\n";
+	printf "   -M (--maxmsgsize) Max message size - useful only for v1 or v2c\n";
 	printf "   -t (--timeout)    seconds before the plugin times out (default=$TIMEOUT)\n";
 	printf "   -V (--version)    Plugin version\n";
 	printf "   -h (--help)       usage help \n\n";

plugins-scripts/check_ifstatus.pl

@@ -280,7 +280,7 @@ sub print_help() {
 	printf "                     in hex with 0x prefix generated by using \"snmpkey\" utility\n"; 
 	printf "                     privacy password and authEngineID\n";
 	printf "   -P (--privproto)  privacy protocol (DES or AES; default: DES)\n";
-	printf "   -M (--maxmsgsize) Max message size - usefull only for v1 or v2c\n";
+	printf "   -M (--maxmsgsize) Max message size - useful only for v1 or v2c\n";
 	printf "   -t (--timeout)    seconds before the plugin times out (default=$TIMEOUT)\n";
 	printf "   -V (--version)    Plugin version\n";
 	printf "   -h (--help)       usage help \n\n";

plugins/check_ntp_time.c

@@ -633,7 +633,7 @@ void print_help(void){
 	printf("%s\n", _("Notes:"));
 	printf(" %s\n", _("If you'd rather want to monitor an NTP server, please use"));
 	printf(" %s\n", _("check_ntp_peer."));
-	printf(" %s\n", _("--time-offset is usefull for compensating for servers with known"));
+	printf(" %s\n", _("--time-offset is useful for compensating for servers with known"));
 	printf(" %s\n", _("and expected clock skew."));
 	printf("\n");
 	printf(UT_THRESHOLDS_NOTES);